Synchronization: Refactor Waiter to allow us to write tests

Instead of being only able to test the platform Waiter implementation,
this allows us to be able to test all Waiter implementations that
build on a specific platform.

A unittest is added that tests all implementations that work for the
platform, and allows us to check that the expected one is being used
by printing the name of the selected implementation.

PiperOrigin-RevId: 518072415
Change-Id: Ie9e6fcd9d8283b4038e6f4e68a304d2adcc04b19

17 files changed, 1089 insertions, 518 deletions

diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
index 01aa2c19..3de1b235 100644
--- a/CMake/AbseilDll.cmake
+++ b/CMake/AbseilDll.cmake
@@ -362,15 +362,24 @@ set(ABSL_INTERNAL_DLL_FILES
   "synchronization/internal/create_thread_identity.cc"
   "synchronization/internal/create_thread_identity.h"
   "synchronization/internal/futex.h"
+  "synchronization/internal/futex_waiter.h"
+  "synchronization/internal/futex_waiter.cc"
   "synchronization/internal/graphcycles.cc"
   "synchronization/internal/graphcycles.h"
   "synchronization/internal/kernel_timeout.h"
   "synchronization/internal/kernel_timeout.cc"
   "synchronization/internal/per_thread_sem.cc"
   "synchronization/internal/per_thread_sem.h"
+  "synchronization/internal/pthread_waiter.h"
+  "synchronization/internal/pthread_waiter.cc"
+  "synchronization/internal/sem_waiter.h"
+  "synchronization/internal/sem_waiter.cc"
   "synchronization/internal/thread_pool.h"
-  "synchronization/internal/waiter.cc"
   "synchronization/internal/waiter.h"
+  "synchronization/internal/waiter_base.h"
+  "synchronization/internal/waiter_base.cc"
+  "synchronization/internal/win32_waiter.h"
+  "synchronization/internal/win32_waiter.cc"
   "time/civil_time.cc"
   "time/civil_time.h"
   "time/clock.cc"
diff --git a/absl/synchronization/BUILD.bazel b/absl/synchronization/BUILD.bazel
index 9c89ac49..40843b35 100644
--- a/absl/synchronization/BUILD.bazel
+++ b/absl/synchronization/BUILD.bazel
@@ -86,8 +86,12 @@ cc_library(
         "barrier.cc",
         "blocking_counter.cc",
         "internal/create_thread_identity.cc",
+        "internal/futex_waiter.cc",
         "internal/per_thread_sem.cc",
-        "internal/waiter.cc",
+        "internal/pthread_waiter.cc",
+        "internal/sem_waiter.cc",
+        "internal/waiter_base.cc",
+        "internal/win32_waiter.cc",
         "mutex.cc",
         "notification.cc",
     ],
@@ -96,8 +100,13 @@ cc_library(
         "blocking_counter.h",
         "internal/create_thread_identity.h",
         "internal/futex.h",
+        "internal/futex_waiter.h",
         "internal/per_thread_sem.h",
+        "internal/pthread_waiter.h",
+        "internal/sem_waiter.h",
         "internal/waiter.h",
+        "internal/waiter_base.h",
+        "internal/win32_waiter.h",
         "mutex.h",
         "notification.h",
     ],
@@ -329,6 +338,20 @@ cc_test(
 )
 
 cc_test(
+    name = "waiter_test",
+    srcs = ["internal/waiter_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":kernel_timeout_internal",
+        ":synchronization",
+        ":thread_pool",
+        "//absl/time",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
     name = "lifetime_test",
     srcs = [
         "lifetime_test.cc",
diff --git a/absl/synchronization/CMakeLists.txt b/absl/synchronization/CMakeLists.txt
index 5eb9b566..d95e327f 100644
--- a/absl/synchronization/CMakeLists.txt
+++ b/absl/synchronization/CMakeLists.txt
@@ -71,16 +71,25 @@ absl_cc_library(
     "blocking_counter.h"
     "internal/create_thread_identity.h"
     "internal/futex.h"
+    "internal/futex_waiter.h"
     "internal/per_thread_sem.h"
+    "internal/pthread_waiter.h"
+    "internal/sem_waiter.h"
     "internal/waiter.h"
+    "internal/waiter_base.h"
+    "internal/win32_waiter.h"
     "mutex.h"
     "notification.h"
   SRCS
     "barrier.cc"
     "blocking_counter.cc"
     "internal/create_thread_identity.cc"
+    "internal/futex_waiter.cc"
     "internal/per_thread_sem.cc"
-    "internal/waiter.cc"
+    "internal/pthread_waiter.cc"
+    "internal/sem_waiter.cc"
+    "internal/waiter_base.cc"
+    "internal/win32_waiter.cc"
     "notification.cc"
     "mutex.cc"
   COPTS
@@ -239,6 +248,21 @@ absl_cc_test(
 
 absl_cc_test(
   NAME
+    waiter_test
+  SRCS
+    "internal/waiter_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::kernel_timeout_internal
+    absl::synchronization
+    absl::thread_pool
+    absl::time
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
     lifetime_test
   SRCS
     "lifetime_test.cc"
diff --git a/absl/synchronization/internal/futex_waiter.cc b/absl/synchronization/internal/futex_waiter.cc
new file mode 100644
index 00000000..0a76bc9a
--- /dev/null
+++ b/absl/synchronization/internal/futex_waiter.cc
@@ -0,0 +1,90 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/futex_waiter.h"
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX_WAITER
+
+#include <atomic>
+#include <cstdint>
+#include <cerrno>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/futex.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr char FutexWaiter::kName[];
+#endif
+
+bool FutexWaiter::Wait(KernelTimeout t) {
+  // Loop until we can atomically decrement futex from a positive
+  // value, waiting on a futex while we believe it is zero.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+
+  while (true) {
+    int32_t x = futex_.load(std::memory_order_relaxed);
+    while (x != 0) {
+      if (!futex_.compare_exchange_weak(x, x - 1,
+                                        std::memory_order_acquire,
+                                        std::memory_order_relaxed)) {
+        continue;  // Raced with someone, retry.
+      }
+      return true;  // Consumed a wakeup, we are done.
+    }
+
+    if (!first_pass) MaybeBecomeIdle();
+    const int err = Futex::WaitUntil(&futex_, 0, t);
+    if (err != 0) {
+      if (err == -EINTR || err == -EWOULDBLOCK) {
+        // Do nothing, the loop will retry.
+      } else if (err == -ETIMEDOUT) {
+        return false;
+      } else {
+        ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
+      }
+    }
+    first_pass = false;
+  }
+}
+
+void FutexWaiter::Post() {
+  if (futex_.fetch_add(1, std::memory_order_release) == 0) {
+    // We incremented from 0, need to wake a potential waiter.
+    Poke();
+  }
+}
+
+void FutexWaiter::Poke() {
+  // Wake one thread waiting on the futex.
+  const int err = Futex::Wake(&futex_, 1);
+  if (ABSL_PREDICT_FALSE(err < 0)) {
+    ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_FUTEX_WAITER
diff --git a/absl/synchronization/internal/futex_waiter.h b/absl/synchronization/internal/futex_waiter.h
new file mode 100644
index 00000000..a6a72095
--- /dev/null
+++ b/absl/synchronization/internal/futex_waiter.h
@@ -0,0 +1,58 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_WAITER_H_
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/futex.h"
+#include "absl/synchronization/internal/waiter_base.h"
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#define ABSL_INTERNAL_HAVE_FUTEX_WAITER 1
+
+class FutexWaiter : public WaiterCrtp<FutexWaiter> {
+ public:
+  FutexWaiter() : futex_(0) {}
+
+  bool Wait(KernelTimeout t);
+  void Post();
+  void Poke();
+
+  static constexpr char kName[] = "FutexWaiter";
+
+ private:
+  // Futexes are defined by specification to be 32-bits.
+  // Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
+  std::atomic<int32_t> futex_;
+  static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_FUTEX
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_WAITER_H_
diff --git a/absl/synchronization/internal/kernel_timeout.h b/absl/synchronization/internal/kernel_timeout.h
index e2cf3c2a..952bd2c6 100644
--- a/absl/synchronization/internal/kernel_timeout.h
+++ b/absl/synchronization/internal/kernel_timeout.h
@@ -30,8 +30,6 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
-class Waiter;
-
 // An optional timeout, with nanosecond granularity.
 //
 // This is a private low-level API for use by a handful of low-level
diff --git a/absl/synchronization/internal/pthread_waiter.cc b/absl/synchronization/internal/pthread_waiter.cc
new file mode 100644
index 00000000..52587827
--- /dev/null
+++ b/absl/synchronization/internal/pthread_waiter.cc
@@ -0,0 +1,138 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/pthread_waiter.h"
+
+#ifdef ABSL_INTERNAL_HAVE_PTHREAD_WAITER
+
+#include <pthread.h>
+#include <sys/time.h>
+#include <unistd.h>
+
+#include <cerrno>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+namespace {
+class PthreadMutexHolder {
+ public:
+  explicit PthreadMutexHolder(pthread_mutex_t *mu) : mu_(mu) {
+    const int err = pthread_mutex_lock(mu_);
+    if (err != 0) {
+      ABSL_RAW_LOG(FATAL, "pthread_mutex_lock failed: %d", err);
+    }
+  }
+
+  PthreadMutexHolder(const PthreadMutexHolder &rhs) = delete;
+  PthreadMutexHolder &operator=(const PthreadMutexHolder &rhs) = delete;
+
+  ~PthreadMutexHolder() {
+    const int err = pthread_mutex_unlock(mu_);
+    if (err != 0) {
+      ABSL_RAW_LOG(FATAL, "pthread_mutex_unlock failed: %d", err);
+    }
+  }
+
+ private:
+  pthread_mutex_t *mu_;
+};
+}  // namespace
+
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr char PthreadWaiter::kName[];
+#endif
+
+PthreadWaiter::PthreadWaiter() : waiter_count_(0), wakeup_count_(0) {
+  const int err = pthread_mutex_init(&mu_, 0);
+  if (err != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_mutex_init failed: %d", err);
+  }
+
+  const int err2 = pthread_cond_init(&cv_, 0);
+  if (err2 != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_cond_init failed: %d", err2);
+  }
+}
+
+bool PthreadWaiter::Wait(KernelTimeout t) {
+  struct timespec abs_timeout;
+  if (t.has_timeout()) {
+    abs_timeout = t.MakeAbsTimespec();
+  }
+
+  PthreadMutexHolder h(&mu_);
+  ++waiter_count_;
+  // Loop until we find a wakeup to consume or timeout.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (wakeup_count_ == 0) {
+    if (!first_pass) MaybeBecomeIdle();
+    // No wakeups available, time to wait.
+    if (!t.has_timeout()) {
+      const int err = pthread_cond_wait(&cv_, &mu_);
+      if (err != 0) {
+        ABSL_RAW_LOG(FATAL, "pthread_cond_wait failed: %d", err);
+      }
+    } else {
+      const int err = pthread_cond_timedwait(&cv_, &mu_, &abs_timeout);
+      if (err == ETIMEDOUT) {
+        --waiter_count_;
+        return false;
+      }
+      if (err != 0) {
+        ABSL_RAW_LOG(FATAL, "pthread_cond_timedwait failed: %d", err);
+      }
+    }
+    first_pass = false;
+  }
+  // Consume a wakeup and we're done.
+  --wakeup_count_;
+  --waiter_count_;
+  return true;
+}
+
+void PthreadWaiter::Post() {
+  PthreadMutexHolder h(&mu_);
+  ++wakeup_count_;
+  InternalCondVarPoke();
+}
+
+void PthreadWaiter::Poke() {
+  PthreadMutexHolder h(&mu_);
+  InternalCondVarPoke();
+}
+
+void PthreadWaiter::InternalCondVarPoke() {
+  if (waiter_count_ != 0) {
+    const int err = pthread_cond_signal(&cv_);
+    if (ABSL_PREDICT_FALSE(err != 0)) {
+      ABSL_RAW_LOG(FATAL, "pthread_cond_signal failed: %d", err);
+    }
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_PTHREAD_WAITER
diff --git a/absl/synchronization/internal/pthread_waiter.h b/absl/synchronization/internal/pthread_waiter.h
new file mode 100644
index 00000000..a930a9e8
--- /dev/null
+++ b/absl/synchronization/internal/pthread_waiter.h
@@ -0,0 +1,58 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_PTHREAD_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_PTHREAD_WAITER_H_
+
+#ifndef _WIN32
+#include <pthread.h>
+
+#include "absl/base/config.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/waiter_base.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#define ABSL_INTERNAL_HAVE_PTHREAD_WAITER 1
+
+class PthreadWaiter : public WaiterCrtp<PthreadWaiter> {
+ public:
+  PthreadWaiter();
+
+  bool Wait(KernelTimeout t);
+  void Post();
+  void Poke();
+
+  static constexpr char kName[] = "PthreadWaiter";
+
+ private:
+  // REQUIRES: mu_ must be held.
+  void InternalCondVarPoke();
+
+  pthread_mutex_t mu_;
+  pthread_cond_t cv_;
+  int waiter_count_;
+  int wakeup_count_;  // Unclaimed wakeups.
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ndef _WIN32
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_PTHREAD_WAITER_H_
diff --git a/absl/synchronization/internal/sem_waiter.cc b/absl/synchronization/internal/sem_waiter.cc
new file mode 100644
index 00000000..89af5de2
--- /dev/null
+++ b/absl/synchronization/internal/sem_waiter.cc
@@ -0,0 +1,104 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/sem_waiter.h"
+
+#ifdef ABSL_INTERNAL_HAVE_SEM_WAITER
+
+#include <semaphore.h>
+
+#include <atomic>
+#include <cassert>
+#include <cstdint>
+#include <cerrno>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr char SemWaiter::kName[];
+#endif
+
+SemWaiter::SemWaiter() : wakeups_(0) {
+  if (sem_init(&sem_, 0, 0) != 0) {
+    ABSL_RAW_LOG(FATAL, "sem_init failed with errno %d\n", errno);
+  }
+}
+
+bool SemWaiter::Wait(KernelTimeout t) {
+  struct timespec abs_timeout;
+  if (t.has_timeout()) {
+    abs_timeout = t.MakeAbsTimespec();
+  }
+
+  // Loop until we timeout or consume a wakeup.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (true) {
+    int x = wakeups_.load(std::memory_order_relaxed);
+    while (x != 0) {
+      if (!wakeups_.compare_exchange_weak(x, x - 1,
+                                          std::memory_order_acquire,
+                                          std::memory_order_relaxed)) {
+        continue;  // Raced with someone, retry.
+      }
+      // Successfully consumed a wakeup, we're done.
+      return true;
+    }
+
+    if (!first_pass) MaybeBecomeIdle();
+    // Nothing to consume, wait (looping on EINTR).
+    while (true) {
+      if (!t.has_timeout()) {
+        if (sem_wait(&sem_) == 0) break;
+        if (errno == EINTR) continue;
+        ABSL_RAW_LOG(FATAL, "sem_wait failed: %d", errno);
+      } else {
+        if (sem_timedwait(&sem_, &abs_timeout) == 0) break;
+        if (errno == EINTR) continue;
+        if (errno == ETIMEDOUT) return false;
+        ABSL_RAW_LOG(FATAL, "sem_timedwait failed: %d", errno);
+      }
+    }
+    first_pass = false;
+  }
+}
+
+void SemWaiter::Post() {
+  // Post a wakeup.
+  if (wakeups_.fetch_add(1, std::memory_order_release) == 0) {
+    // We incremented from 0, need to wake a potential waiter.
+    Poke();
+  }
+}
+
+void SemWaiter::Poke() {
+  if (sem_post(&sem_) != 0) {  // Wake any semaphore waiter.
+    ABSL_RAW_LOG(FATAL, "sem_post failed with errno %d\n", errno);
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_SEM_WAITER
diff --git a/absl/synchronization/internal/sem_waiter.h b/absl/synchronization/internal/sem_waiter.h
new file mode 100644
index 00000000..47d5bf3d
--- /dev/null
+++ b/absl/synchronization/internal/sem_waiter.h
@@ -0,0 +1,63 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_SEM_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_SEM_WAITER_H_
+
+#include "absl/base/config.h"
+
+#ifdef ABSL_HAVE_SEMAPHORE_H
+#include <semaphore.h>
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/internal/thread_identity.h"
+#include "absl/synchronization/internal/futex.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/waiter_base.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#define ABSL_INTERNAL_HAVE_SEM_WAITER 1
+
+class SemWaiter : public WaiterCrtp<SemWaiter> {
+ public:
+  SemWaiter();
+
+  bool Wait(KernelTimeout t);
+  void Post();
+  void Poke();
+
+  static constexpr char kName[] = "SemWaiter";
+
+ private:
+  sem_t sem_;
+
+  // This seems superfluous, but for Poke() we need to cause spurious
+  // wakeups on the semaphore. Hence we can't actually use the
+  // semaphore's count.
+  std::atomic<int> wakeups_;
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_HAVE_SEMAPHORE_H
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_SEM_WAITER_H_
diff --git a/absl/synchronization/internal/waiter.cc b/absl/synchronization/internal/waiter.cc
deleted file mode 100644
index 1b8d8d09..00000000
--- a/absl/synchronization/internal/waiter.cc
+++ /dev/null
@@ -1,400 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/synchronization/internal/waiter.h"
-
-#include "absl/base/config.h"
-
-#ifdef _WIN32
-#include <windows.h>
-#else
-#include <pthread.h>
-#include <sys/time.h>
-#include <unistd.h>
-#endif
-
-#ifdef __linux__
-#include <linux/futex.h>
-#include <sys/syscall.h>
-#endif
-
-#ifdef ABSL_HAVE_SEMAPHORE_H
-#include <semaphore.h>
-#endif
-
-#include <errno.h>
-#include <stdio.h>
-#include <time.h>
-
-#include <atomic>
-#include <cassert>
-#include <cstdint>
-#include <new>
-#include <type_traits>
-
-#include "absl/base/internal/raw_logging.h"
-#include "absl/base/internal/thread_identity.h"
-#include "absl/base/optimization.h"
-#include "absl/synchronization/internal/kernel_timeout.h"
-
-namespace absl {
-ABSL_NAMESPACE_BEGIN
-namespace synchronization_internal {
-
-static void MaybeBecomeIdle() {
-  base_internal::ThreadIdentity *identity =
-      base_internal::CurrentThreadIdentityIfPresent();
-  assert(identity != nullptr);
-  const bool is_idle = identity->is_idle.load(std::memory_order_relaxed);
-  const int ticker = identity->ticker.load(std::memory_order_relaxed);
-  const int wait_start = identity->wait_start.load(std::memory_order_relaxed);
-  if (!is_idle && ticker - wait_start > Waiter::kIdlePeriods) {
-    identity->is_idle.store(true, std::memory_order_relaxed);
-  }
-}
-
-#if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
-
-Waiter::Waiter() : futex_(0) {}
-
-bool Waiter::Wait(KernelTimeout t) {
-  // Loop until we can atomically decrement futex from a positive
-  // value, waiting on a futex while we believe it is zero.
-  // Note that, since the thread ticker is just reset, we don't need to check
-  // whether the thread is idle on the very first pass of the loop.
-  bool first_pass = true;
-
-  while (true) {
-    int32_t x = futex_.load(std::memory_order_relaxed);
-    while (x != 0) {
-      if (!futex_.compare_exchange_weak(x, x - 1,
-                                        std::memory_order_acquire,
-                                        std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      return true;  // Consumed a wakeup, we are done.
-    }
-
-    if (!first_pass) MaybeBecomeIdle();
-    const int err = Futex::WaitUntil(&futex_, 0, t);
-    if (err != 0) {
-      if (err == -EINTR || err == -EWOULDBLOCK) {
-        // Do nothing, the loop will retry.
-      } else if (err == -ETIMEDOUT) {
-        return false;
-      } else {
-        ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
-      }
-    }
-    first_pass = false;
-  }
-}
-
-void Waiter::Post() {
-  if (futex_.fetch_add(1, std::memory_order_release) == 0) {
-    // We incremented from 0, need to wake a potential waiter.
-    Poke();
-  }
-}
-
-void Waiter::Poke() {
-  // Wake one thread waiting on the futex.
-  const int err = Futex::Wake(&futex_, 1);
-  if (ABSL_PREDICT_FALSE(err < 0)) {
-    ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
-  }
-}
-
-#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
-
-class PthreadMutexHolder {
- public:
-  explicit PthreadMutexHolder(pthread_mutex_t *mu) : mu_(mu) {
-    const int err = pthread_mutex_lock(mu_);
-    if (err != 0) {
-      ABSL_RAW_LOG(FATAL, "pthread_mutex_lock failed: %d", err);
-    }
-  }
-
-  PthreadMutexHolder(const PthreadMutexHolder &rhs) = delete;
-  PthreadMutexHolder &operator=(const PthreadMutexHolder &rhs) = delete;
-
-  ~PthreadMutexHolder() {
-    const int err = pthread_mutex_unlock(mu_);
-    if (err != 0) {
-      ABSL_RAW_LOG(FATAL, "pthread_mutex_unlock failed: %d", err);
-    }
-  }
-
- private:
-  pthread_mutex_t *mu_;
-};
-
-Waiter::Waiter() {
-  const int err = pthread_mutex_init(&mu_, 0);
-  if (err != 0) {
-    ABSL_RAW_LOG(FATAL, "pthread_mutex_init failed: %d", err);
-  }
-
-  const int err2 = pthread_cond_init(&cv_, 0);
-  if (err2 != 0) {
-    ABSL_RAW_LOG(FATAL, "pthread_cond_init failed: %d", err2);
-  }
-
-  waiter_count_ = 0;
-  wakeup_count_ = 0;
-}
-
-bool Waiter::Wait(KernelTimeout t) {
-  struct timespec abs_timeout;
-  if (t.has_timeout()) {
-    abs_timeout = t.MakeAbsTimespec();
-  }
-
-  PthreadMutexHolder h(&mu_);
-  ++waiter_count_;
-  // Loop until we find a wakeup to consume or timeout.
-  // Note that, since the thread ticker is just reset, we don't need to check
-  // whether the thread is idle on the very first pass of the loop.
-  bool first_pass = true;
-  while (wakeup_count_ == 0) {
-    if (!first_pass) MaybeBecomeIdle();
-    // No wakeups available, time to wait.
-    if (!t.has_timeout()) {
-      const int err = pthread_cond_wait(&cv_, &mu_);
-      if (err != 0) {
-        ABSL_RAW_LOG(FATAL, "pthread_cond_wait failed: %d", err);
-      }
-    } else {
-      const int err = pthread_cond_timedwait(&cv_, &mu_, &abs_timeout);
-      if (err == ETIMEDOUT) {
-        --waiter_count_;
-        return false;
-      }
-      if (err != 0) {
-        ABSL_RAW_LOG(FATAL, "pthread_cond_timedwait failed: %d", err);
-      }
-    }
-    first_pass = false;
-  }
-  // Consume a wakeup and we're done.
-  --wakeup_count_;
-  --waiter_count_;
-  return true;
-}
-
-void Waiter::Post() {
-  PthreadMutexHolder h(&mu_);
-  ++wakeup_count_;
-  InternalCondVarPoke();
-}
-
-void Waiter::Poke() {
-  PthreadMutexHolder h(&mu_);
-  InternalCondVarPoke();
-}
-
-void Waiter::InternalCondVarPoke() {
-  if (waiter_count_ != 0) {
-    const int err = pthread_cond_signal(&cv_);
-    if (ABSL_PREDICT_FALSE(err != 0)) {
-      ABSL_RAW_LOG(FATAL, "pthread_cond_signal failed: %d", err);
-    }
-  }
-}
-
-#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
-
-Waiter::Waiter() {
-  if (sem_init(&sem_, 0, 0) != 0) {
-    ABSL_RAW_LOG(FATAL, "sem_init failed with errno %d\n", errno);
-  }
-  wakeups_.store(0, std::memory_order_relaxed);
-}
-
-bool Waiter::Wait(KernelTimeout t) {
-  struct timespec abs_timeout;
-  if (t.has_timeout()) {
-    abs_timeout = t.MakeAbsTimespec();
-  }
-
-  // Loop until we timeout or consume a wakeup.
-  // Note that, since the thread ticker is just reset, we don't need to check
-  // whether the thread is idle on the very first pass of the loop.
-  bool first_pass = true;
-  while (true) {
-    int x = wakeups_.load(std::memory_order_relaxed);
-    while (x != 0) {
-      if (!wakeups_.compare_exchange_weak(x, x - 1,
-                                          std::memory_order_acquire,
-                                          std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      // Successfully consumed a wakeup, we're done.
-      return true;
-    }
-
-    if (!first_pass) MaybeBecomeIdle();
-    // Nothing to consume, wait (looping on EINTR).
-    while (true) {
-      if (!t.has_timeout()) {
-        if (sem_wait(&sem_) == 0) break;
-        if (errno == EINTR) continue;
-        ABSL_RAW_LOG(FATAL, "sem_wait failed: %d", errno);
-      } else {
-        if (sem_timedwait(&sem_, &abs_timeout) == 0) break;
-        if (errno == EINTR) continue;
-        if (errno == ETIMEDOUT) return false;
-        ABSL_RAW_LOG(FATAL, "sem_timedwait failed: %d", errno);
-      }
-    }
-    first_pass = false;
-  }
-}
-
-void Waiter::Post() {
-  // Post a wakeup.
-  if (wakeups_.fetch_add(1, std::memory_order_release) == 0) {
-    // We incremented from 0, need to wake a potential waiter.
-    Poke();
-  }
-}
-
-void Waiter::Poke() {
-  if (sem_post(&sem_) != 0) {  // Wake any semaphore waiter.
-    ABSL_RAW_LOG(FATAL, "sem_post failed with errno %d\n", errno);
-  }
-}
-
-#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
-
-class Waiter::WinHelper {
- public:
-  static SRWLOCK *GetLock(Waiter *w) {
-    return reinterpret_cast<SRWLOCK *>(&w->mu_storage_);
-  }
-
-  static CONDITION_VARIABLE *GetCond(Waiter *w) {
-    return reinterpret_cast<CONDITION_VARIABLE *>(&w->cv_storage_);
-  }
-
-  static_assert(sizeof(SRWLOCK) == sizeof(void *),
-                "`mu_storage_` does not have the same size as SRWLOCK");
-  static_assert(alignof(SRWLOCK) == alignof(void *),
-                "`mu_storage_` does not have the same alignment as SRWLOCK");
-
-  static_assert(sizeof(CONDITION_VARIABLE) == sizeof(void *),
-                "`ABSL_CONDITION_VARIABLE_STORAGE` does not have the same size "
-                "as `CONDITION_VARIABLE`");
-  static_assert(
-      alignof(CONDITION_VARIABLE) == alignof(void *),
-      "`cv_storage_` does not have the same alignment as `CONDITION_VARIABLE`");
-
-  // The SRWLOCK and CONDITION_VARIABLE types must be trivially constructible
-  // and destructible because we never call their constructors or destructors.
-  static_assert(std::is_trivially_constructible<SRWLOCK>::value,
-                "The `SRWLOCK` type must be trivially constructible");
-  static_assert(
-      std::is_trivially_constructible<CONDITION_VARIABLE>::value,
-      "The `CONDITION_VARIABLE` type must be trivially constructible");
-  static_assert(std::is_trivially_destructible<SRWLOCK>::value,
-                "The `SRWLOCK` type must be trivially destructible");
-  static_assert(std::is_trivially_destructible<CONDITION_VARIABLE>::value,
-                "The `CONDITION_VARIABLE` type must be trivially destructible");
-};
-
-class LockHolder {
- public:
-  explicit LockHolder(SRWLOCK* mu) : mu_(mu) {
-    AcquireSRWLockExclusive(mu_);
-  }
-
-  LockHolder(const LockHolder&) = delete;
-  LockHolder& operator=(const LockHolder&) = delete;
-
-  ~LockHolder() {
-    ReleaseSRWLockExclusive(mu_);
-  }
-
- private:
-  SRWLOCK* mu_;
-};
-
-Waiter::Waiter() {
-  auto *mu = ::new (static_cast<void *>(&mu_storage_)) SRWLOCK;
-  auto *cv = ::new (static_cast<void *>(&cv_storage_)) CONDITION_VARIABLE;
-  InitializeSRWLock(mu);
-  InitializeConditionVariable(cv);
-  waiter_count_ = 0;
-  wakeup_count_ = 0;
-}
-
-bool Waiter::Wait(KernelTimeout t) {
-  SRWLOCK *mu = WinHelper::GetLock(this);
-  CONDITION_VARIABLE *cv = WinHelper::GetCond(this);
-
-  LockHolder h(mu);
-  ++waiter_count_;
-
-  // Loop until we find a wakeup to consume or timeout.
-  // Note that, since the thread ticker is just reset, we don't need to check
-  // whether the thread is idle on the very first pass of the loop.
-  bool first_pass = true;
-  while (wakeup_count_ == 0) {
-    if (!first_pass) MaybeBecomeIdle();
-    // No wakeups available, time to wait.
-    if (!SleepConditionVariableSRW(cv, mu, t.InMillisecondsFromNow(), 0)) {
-      // GetLastError() returns a Win32 DWORD, but we assign to
-      // unsigned long to simplify the ABSL_RAW_LOG case below.  The uniform
-      // initialization guarantees this is not a narrowing conversion.
-      const unsigned long err{GetLastError()};  // NOLINT(runtime/int)
-      if (err == ERROR_TIMEOUT) {
-        --waiter_count_;
-        return false;
-      } else {
-        ABSL_RAW_LOG(FATAL, "SleepConditionVariableSRW failed: %lu", err);
-      }
-    }
-    first_pass = false;
-  }
-  // Consume a wakeup and we're done.
-  --wakeup_count_;
-  --waiter_count_;
-  return true;
-}
-
-void Waiter::Post() {
-  LockHolder h(WinHelper::GetLock(this));
-  ++wakeup_count_;
-  InternalCondVarPoke();
-}
-
-void Waiter::Poke() {
-  LockHolder h(WinHelper::GetLock(this));
-  InternalCondVarPoke();
-}
-
-void Waiter::InternalCondVarPoke() {
-  if (waiter_count_ != 0) {
-    WakeConditionVariable(WinHelper::GetCond(this));
-  }
-}
-
-#else
-#error Unknown ABSL_WAITER_MODE
-#endif
-
-}  // namespace synchronization_internal
-ABSL_NAMESPACE_END
-}  // namespace absl
diff --git a/absl/synchronization/internal/waiter.h b/absl/synchronization/internal/waiter.h
index b8adfeb5..07bba10a 100644
--- a/absl/synchronization/internal/waiter.h
+++ b/absl/synchronization/internal/waiter.h
@@ -17,27 +17,10 @@
 #define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
 
 #include "absl/base/config.h"
-
-#ifdef _WIN32
-#include <sdkddkver.h>
-#else
-#include <pthread.h>
-#endif
-
-#ifdef __linux__
-#include <linux/futex.h>
-#endif
-
-#ifdef ABSL_HAVE_SEMAPHORE_H
-#include <semaphore.h>
-#endif
-
-#include <atomic>
-#include <cstdint>
-
-#include "absl/base/internal/thread_identity.h"
-#include "absl/synchronization/internal/futex.h"
-#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/futex_waiter.h"
+#include "absl/synchronization/internal/pthread_waiter.h"
+#include "absl/synchronization/internal/sem_waiter.h"
+#include "absl/synchronization/internal/win32_waiter.h"
 
 // May be chosen at compile time via -DABSL_FORCE_WAITER_MODE=<index>
 #define ABSL_WAITER_MODE_FUTEX 0
@@ -47,112 +30,31 @@
 
 #if defined(ABSL_FORCE_WAITER_MODE)
 #define ABSL_WAITER_MODE ABSL_FORCE_WAITER_MODE
-#elif defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
+#elif defined(ABSL_INTERNAL_HAVE_WIN32_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_WIN32
-#elif defined(ABSL_INTERNAL_HAVE_FUTEX)
+#elif defined(ABSL_INTERNAL_HAVE_FUTEX_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
-#elif defined(ABSL_HAVE_SEMAPHORE_H)
+#elif defined(ABSL_INTERNAL_HAVE_SEM_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_SEM
-#else
+#elif defined(ABSL_INTERNAL_HAVE_PTHREAD_WAITER)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_CONDVAR
+#else
+#error ABSL_WAITER_MODE is undefined
 #endif
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
-// Waiter is an OS-specific semaphore.
-class Waiter {
- public:
-  // Prepare any data to track waits.
-  Waiter();
-
-  // Not copyable or movable
-  Waiter(const Waiter&) = delete;
-  Waiter& operator=(const Waiter&) = delete;
-
-  // Blocks the calling thread until a matching call to `Post()` or
-  // `t` has passed. Returns `true` if woken (`Post()` called),
-  // `false` on timeout.
-  bool Wait(KernelTimeout t);
-
-  // Restart the caller of `Wait()` as with a normal semaphore.
-  void Post();
-
-  // If anyone is waiting, wake them up temporarily and cause them to
-  // call `MaybeBecomeIdle()`. They will then return to waiting for a
-  // `Post()` or timeout.
-  void Poke();
-
-  // Returns the Waiter associated with the identity.
-  static Waiter* GetWaiter(base_internal::ThreadIdentity* identity) {
-    static_assert(
-        sizeof(Waiter) <= sizeof(base_internal::ThreadIdentity::WaiterState),
-        "Insufficient space for Waiter");
-    return reinterpret_cast<Waiter*>(identity->waiter_state.data);
-  }
-
-  // How many periods to remain idle before releasing resources
-#ifndef ABSL_HAVE_THREAD_SANITIZER
-  static constexpr int kIdlePeriods = 60;
-#else
-  // Memory consumption under ThreadSanitizer is a serious concern,
-  // so we release resources sooner. The value of 1 leads to 1 to 2 second
-  // delay before marking a thread as idle.
-  static const int kIdlePeriods = 1;
-#endif
-
- private:
-  // The destructor must not be called since Mutex/CondVar
-  // can use PerThreadSem/Waiter after the thread exits.
-  // Waiter objects are embedded in ThreadIdentity objects,
-  // which are reused via a freelist and are never destroyed.
-  ~Waiter() = delete;
-
 #if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
-  // Futexes are defined by specification to be 32-bits.
-  // Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
-  std::atomic<int32_t> futex_;
-  static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");
-
-#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
-  // REQUIRES: mu_ must be held.
-  void InternalCondVarPoke();
-
-  pthread_mutex_t mu_;
-  pthread_cond_t cv_;
-  int waiter_count_;
-  int wakeup_count_;  // Unclaimed wakeups.
-
+using Waiter = FutexWaiter;
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
-  sem_t sem_;
-  // This seems superfluous, but for Poke() we need to cause spurious
-  // wakeups on the semaphore. Hence we can't actually use the
-  // semaphore's count.
-  std::atomic<int> wakeups_;
-
+using Waiter = SemWaiter;
+#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
+using Waiter = PthreadWaiter;
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
-  // WinHelper - Used to define utilities for accessing the lock and
-  // condition variable storage once the types are complete.
-  class WinHelper;
-
-  // REQUIRES: WinHelper::GetLock(this) must be held.
-  void InternalCondVarPoke();
-
-  // We can't include Windows.h in our headers, so we use aligned character
-  // buffers to define the storage of SRWLOCK and CONDITION_VARIABLE.
-  // SRW locks and condition variables do not need to be explicitly destroyed.
-  // https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
-  // https://stackoverflow.com/questions/28975958/why-does-windows-have-no-deleteconditionvariable-function-to-go-together-with
-  alignas(void*) unsigned char mu_storage_[sizeof(void*)];
-  alignas(void*) unsigned char cv_storage_[sizeof(void*)];
-  int waiter_count_;
-  int wakeup_count_;
-
-#else
-  #error Unknown ABSL_WAITER_MODE
+using Waiter = Win32Waiter;
 #endif
-};
 
 }  // namespace synchronization_internal
 ABSL_NAMESPACE_END
diff --git a/absl/synchronization/internal/waiter_base.cc b/absl/synchronization/internal/waiter_base.cc
new file mode 100644
index 00000000..46928b40
--- /dev/null
+++ b/absl/synchronization/internal/waiter_base.cc
@@ -0,0 +1,42 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/waiter_base.h"
+
+#include "absl/base/config.h"
+#include "absl/base/internal/thread_identity.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr int WaiterBase::kIdlePeriods;
+#endif
+
+void WaiterBase::MaybeBecomeIdle() {
+  base_internal::ThreadIdentity *identity =
+      base_internal::CurrentThreadIdentityIfPresent();
+  assert(identity != nullptr);
+  const bool is_idle = identity->is_idle.load(std::memory_order_relaxed);
+  const int ticker = identity->ticker.load(std::memory_order_relaxed);
+  const int wait_start = identity->wait_start.load(std::memory_order_relaxed);
+  if (!is_idle && ticker - wait_start > kIdlePeriods) {
+    identity->is_idle.store(true, std::memory_order_relaxed);
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/synchronization/internal/waiter_base.h b/absl/synchronization/internal/waiter_base.h
new file mode 100644
index 00000000..cf175481
--- /dev/null
+++ b/absl/synchronization/internal/waiter_base.h
@@ -0,0 +1,90 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_WAITER_BASE_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_BASE_H_
+
+#include "absl/base/config.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+// `Waiter` is a platform specific semaphore implementation that `PerThreadSem`
+// waits on to implement blocking in `absl::Mutex`.  Implementations should
+// inherit from `WaiterCrtp` and must implement `Wait()`, `Post()`, and `Poke()`
+// as described in `WaiterBase`.  `waiter.h` selects the implementation and uses
+// static-dispatch for performance.
+class WaiterBase {
+ public:
+  WaiterBase() = default;
+
+  // Not copyable or movable
+  WaiterBase(const WaiterBase&) = delete;
+  WaiterBase& operator=(const WaiterBase&) = delete;
+
+  // Blocks the calling thread until a matching call to `Post()` or
+  // `t` has passed. Returns `true` if woken (`Post()` called),
+  // `false` on timeout.
+  //
+  // bool Wait(KernelTimeout t);
+
+  // Restart the caller of `Wait()` as with a normal semaphore.
+  //
+  // void Post();
+
+  // If anyone is waiting, wake them up temporarily and cause them to
+  // call `MaybeBecomeIdle()`. They will then return to waiting for a
+  // `Post()` or timeout.
+  //
+  // void Poke();
+
+  // Returns the name of this implementation. Used only for debugging.
+  //
+  // static constexpr char kName[];
+
+  // How many periods to remain idle before releasing resources
+#ifndef ABSL_HAVE_THREAD_SANITIZER
+  static constexpr int kIdlePeriods = 60;
+#else
+  // Memory consumption under ThreadSanitizer is a serious concern,
+  // so we release resources sooner. The value of 1 leads to 1 to 2 second
+  // delay before marking a thread as idle.
+  static constexpr int kIdlePeriods = 1;
+#endif
+
+ protected:
+  static void MaybeBecomeIdle();
+};
+
+template <typename T>
+class WaiterCrtp : public WaiterBase {
+ public:
+  // Returns the Waiter associated with the identity.
+  static T* GetWaiter(base_internal::ThreadIdentity* identity) {
+    static_assert(
+        sizeof(T) <= sizeof(base_internal::ThreadIdentity::WaiterState),
+        "Insufficient space for Waiter");
+    return reinterpret_cast<T*>(identity->waiter_state.data);
+  }
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_BASE_H_
diff --git a/absl/synchronization/internal/waiter_test.cc b/absl/synchronization/internal/waiter_test.cc
new file mode 100644
index 00000000..92812965
--- /dev/null
+++ b/absl/synchronization/internal/waiter_test.cc
@@ -0,0 +1,151 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/waiter.h"
+
+#include <iostream>
+#include <ostream>
+
+#include "absl/synchronization/internal/create_thread_identity.h"
+#include "absl/synchronization/internal/futex_waiter.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/pthread_waiter.h"
+#include "absl/synchronization/internal/sem_waiter.h"
+#include "absl/synchronization/internal/thread_pool.h"
+#include "absl/synchronization/internal/win32_waiter.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+#include "gtest/gtest.h"
+
+namespace {
+
+TEST(Waiter, PrintPlatformImplementation) {
+  // Allows us to verify that the platform is using the expected implementation.
+  std::cout << absl::synchronization_internal::Waiter::kName << std::endl;
+}
+
+template <typename T>
+class WaiterTest : public ::testing::Test {
+ public:
+  // Waiter implementations assume that a ThreadIdentity has already been
+  // created.
+  WaiterTest() {
+    absl::synchronization_internal::GetOrCreateCurrentThreadIdentity();
+  }
+};
+
+TYPED_TEST_SUITE_P(WaiterTest);
+
+constexpr absl::Duration slop = absl::Milliseconds(2);
+
+TYPED_TEST_P(WaiterTest, WaitNoTimeout) {
+  absl::synchronization_internal::ThreadPool tp(1);
+  TypeParam waiter;
+  tp.Schedule([&]() {
+    // Include some `Poke()` calls to ensure they don't cause `waiter` to return
+    // from `Wait()`.
+    waiter.Poke();
+    absl::SleepFor(absl::Seconds(1));
+    waiter.Poke();
+    absl::SleepFor(absl::Seconds(1));
+    waiter.Post();
+  });
+  absl::Time start = absl::Now();
+  EXPECT_TRUE(
+      waiter.Wait(absl::synchronization_internal::KernelTimeout::Never()));
+  absl::Duration waited = absl::Now() - start;
+  EXPECT_GE(waited, absl::Seconds(2) - slop);
+}
+
+TYPED_TEST_P(WaiterTest, WaitDurationWoken) {
+  absl::synchronization_internal::ThreadPool tp(1);
+  TypeParam waiter;
+  tp.Schedule([&]() {
+    // Include some `Poke()` calls to ensure they don't cause `waiter` to return
+    // from `Wait()`.
+    waiter.Poke();
+    absl::SleepFor(absl::Milliseconds(500));
+    waiter.Post();
+  });
+  absl::Time start = absl::Now();
+  EXPECT_TRUE(waiter.Wait(
+      absl::synchronization_internal::KernelTimeout(absl::Seconds(10))));
+  absl::Duration waited = absl::Now() - start;
+  EXPECT_GE(waited, absl::Milliseconds(500) - slop);
+  EXPECT_LT(waited, absl::Seconds(1));
+}
+
+TYPED_TEST_P(WaiterTest, WaitTimeWoken) {
+  absl::synchronization_internal::ThreadPool tp(1);
+  TypeParam waiter;
+  tp.Schedule([&]() {
+    // Include some `Poke()` calls to ensure they don't cause `waiter` to return
+    // from `Wait()`.
+    waiter.Poke();
+    absl::SleepFor(absl::Milliseconds(500));
+    waiter.Post();
+  });
+  absl::Time start = absl::Now();
+  EXPECT_TRUE(waiter.Wait(absl::synchronization_internal::KernelTimeout(
+      start + absl::Seconds(10))));
+  absl::Duration waited = absl::Now() - start;
+  EXPECT_GE(waited, absl::Milliseconds(500) - slop);
+  EXPECT_LT(waited, absl::Seconds(1));
+}
+
+TYPED_TEST_P(WaiterTest, WaitDurationReached) {
+  TypeParam waiter;
+  absl::Time start = absl::Now();
+  EXPECT_FALSE(waiter.Wait(
+      absl::synchronization_internal::KernelTimeout(absl::Milliseconds(500))));
+  absl::Duration waited = absl::Now() - start;
+  EXPECT_GE(waited, absl::Milliseconds(500) - slop);
+  EXPECT_LT(waited, absl::Seconds(1));
+}
+
+TYPED_TEST_P(WaiterTest, WaitTimeReached) {
+  TypeParam waiter;
+  absl::Time start = absl::Now();
+  EXPECT_FALSE(waiter.Wait(absl::synchronization_internal::KernelTimeout(
+      start + absl::Milliseconds(500))));
+  absl::Duration waited = absl::Now() - start;
+  EXPECT_GE(waited, absl::Milliseconds(500) - slop);
+  EXPECT_LT(waited, absl::Seconds(1));
+}
+
+REGISTER_TYPED_TEST_SUITE_P(WaiterTest,
+                            WaitNoTimeout,
+                            WaitDurationWoken,
+                            WaitTimeWoken,
+                            WaitDurationReached,
+                            WaitTimeReached);
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX_WAITER
+INSTANTIATE_TYPED_TEST_SUITE_P(Futex, WaiterTest,
+                               absl::synchronization_internal::FutexWaiter);
+#endif
+#ifdef ABSL_INTERNAL_HAVE_PTHREAD_WAITER
+INSTANTIATE_TYPED_TEST_SUITE_P(Pthread, WaiterTest,
+                               absl::synchronization_internal::PthreadWaiter);
+#endif
+#ifdef ABSL_INTERNAL_HAVE_SEM_WAITER
+INSTANTIATE_TYPED_TEST_SUITE_P(Sem, WaiterTest,
+                               absl::synchronization_internal::SemWaiter);
+#endif
+#ifdef ABSL_INTERNAL_HAVE_WIN32_WAITER
+INSTANTIATE_TYPED_TEST_SUITE_P(Win32, WaiterTest,
+                               absl::synchronization_internal::Win32Waiter);
+#endif
+
+}  // namespace
diff --git a/absl/synchronization/internal/win32_waiter.cc b/absl/synchronization/internal/win32_waiter.cc
new file mode 100644
index 00000000..bd95ff08
--- /dev/null
+++ b/absl/synchronization/internal/win32_waiter.cc
@@ -0,0 +1,151 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/synchronization/internal/win32_waiter.h"
+
+#ifdef ABSL_INTERNAL_HAVE_WIN32_WAITER
+
+#include <windows.h>
+
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr char Win32Waiter::kName[];
+#endif
+
+class Win32Waiter::WinHelper {
+ public:
+  static SRWLOCK *GetLock(Win32Waiter *w) {
+    return reinterpret_cast<SRWLOCK *>(&w->mu_storage_);
+  }
+
+  static CONDITION_VARIABLE *GetCond(Win32Waiter *w) {
+    return reinterpret_cast<CONDITION_VARIABLE *>(&w->cv_storage_);
+  }
+
+  static_assert(sizeof(SRWLOCK) == sizeof(void *),
+                "`mu_storage_` does not have the same size as SRWLOCK");
+  static_assert(alignof(SRWLOCK) == alignof(void *),
+                "`mu_storage_` does not have the same alignment as SRWLOCK");
+
+  static_assert(sizeof(CONDITION_VARIABLE) == sizeof(void *),
+                "`ABSL_CONDITION_VARIABLE_STORAGE` does not have the same size "
+                "as `CONDITION_VARIABLE`");
+  static_assert(
+      alignof(CONDITION_VARIABLE) == alignof(void *),
+      "`cv_storage_` does not have the same alignment as `CONDITION_VARIABLE`");
+
+  // The SRWLOCK and CONDITION_VARIABLE types must be trivially constructible
+  // and destructible because we never call their constructors or destructors.
+  static_assert(std::is_trivially_constructible<SRWLOCK>::value,
+                "The `SRWLOCK` type must be trivially constructible");
+  static_assert(
+      std::is_trivially_constructible<CONDITION_VARIABLE>::value,
+      "The `CONDITION_VARIABLE` type must be trivially constructible");
+  static_assert(std::is_trivially_destructible<SRWLOCK>::value,
+                "The `SRWLOCK` type must be trivially destructible");
+  static_assert(std::is_trivially_destructible<CONDITION_VARIABLE>::value,
+                "The `CONDITION_VARIABLE` type must be trivially destructible");
+};
+
+class LockHolder {
+ public:
+  explicit LockHolder(SRWLOCK* mu) : mu_(mu) {
+    AcquireSRWLockExclusive(mu_);
+  }
+
+  LockHolder(const LockHolder&) = delete;
+  LockHolder& operator=(const LockHolder&) = delete;
+
+  ~LockHolder() {
+    ReleaseSRWLockExclusive(mu_);
+  }
+
+ private:
+  SRWLOCK* mu_;
+};
+
+Win32Waiter::Win32Waiter() {
+  auto *mu = ::new (static_cast<void *>(&mu_storage_)) SRWLOCK;
+  auto *cv = ::new (static_cast<void *>(&cv_storage_)) CONDITION_VARIABLE;
+  InitializeSRWLock(mu);
+  InitializeConditionVariable(cv);
+  waiter_count_ = 0;
+  wakeup_count_ = 0;
+}
+
+bool Win32Waiter::Wait(KernelTimeout t) {
+  SRWLOCK *mu = WinHelper::GetLock(this);
+  CONDITION_VARIABLE *cv = WinHelper::GetCond(this);
+
+  LockHolder h(mu);
+  ++waiter_count_;
+
+  // Loop until we find a wakeup to consume or timeout.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (wakeup_count_ == 0) {
+    if (!first_pass) MaybeBecomeIdle();
+    // No wakeups available, time to wait.
+    if (!SleepConditionVariableSRW(cv, mu, t.InMillisecondsFromNow(), 0)) {
+      // GetLastError() returns a Win32 DWORD, but we assign to
+      // unsigned long to simplify the ABSL_RAW_LOG case below.  The uniform
+      // initialization guarantees this is not a narrowing conversion.
+      const unsigned long err{GetLastError()};  // NOLINT(runtime/int)
+      if (err == ERROR_TIMEOUT) {
+        --waiter_count_;
+        return false;
+      } else {
+        ABSL_RAW_LOG(FATAL, "SleepConditionVariableSRW failed: %lu", err);
+      }
+    }
+    first_pass = false;
+  }
+  // Consume a wakeup and we're done.
+  --wakeup_count_;
+  --waiter_count_;
+  return true;
+}
+
+void Win32Waiter::Post() {
+  LockHolder h(WinHelper::GetLock(this));
+  ++wakeup_count_;
+  InternalCondVarPoke();
+}
+
+void Win32Waiter::Poke() {
+  LockHolder h(WinHelper::GetLock(this));
+  InternalCondVarPoke();
+}
+
+void Win32Waiter::InternalCondVarPoke() {
+  if (waiter_count_ != 0) {
+    WakeConditionVariable(WinHelper::GetCond(this));
+  }
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_WIN32_WAITER
diff --git a/absl/synchronization/internal/win32_waiter.h b/absl/synchronization/internal/win32_waiter.h
new file mode 100644
index 00000000..87eb617c
--- /dev/null
+++ b/absl/synchronization/internal/win32_waiter.h
@@ -0,0 +1,70 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_WIN32_WAITER_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_WIN32_WAITER_H_
+
+#ifdef _WIN32
+#include <sdkddkver.h>
+#endif
+
+#if defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
+
+#include "absl/base/config.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+#include "absl/synchronization/internal/waiter_base.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#define ABSL_INTERNAL_HAVE_WIN32_WAITER 1
+
+class Win32Waiter : public WaiterCrtp<Win32Waiter> {
+ public:
+  Win32Waiter();
+
+  bool Wait(KernelTimeout t);
+  void Post();
+  void Poke();
+
+  static constexpr char kName[] = "Win32Waiter";
+
+ private:
+  // WinHelper - Used to define utilities for accessing the lock and
+  // condition variable storage once the types are complete.
+  class WinHelper;
+
+  // REQUIRES: WinHelper::GetLock(this) must be held.
+  void InternalCondVarPoke();
+
+  // We can't include Windows.h in our headers, so we use aligned character
+  // buffers to define the storage of SRWLOCK and CONDITION_VARIABLE.
+  // SRW locks and condition variables do not need to be explicitly destroyed.
+  // https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
+  // https://stackoverflow.com/questions/28975958/why-does-windows-have-no-deleteconditionvariable-function-to-go-together-with
+  alignas(void*) unsigned char mu_storage_[sizeof(void*)];
+  alignas(void*) unsigned char cv_storage_[sizeof(void*)];
+  int waiter_count_;
+  int wakeup_count_;
+};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_WIN32_WAITER_H_